Water-based paint formulated by blending base resin such as acrylic resin or polyester resin having hydrophilic functional groups such as hydroxyl group, carboxyl group and the like, with melamine resin as crosslinking agent has heretofore been frequently used as intermediate coat, top coat or the like for coating outer panels of automobiles.
As the melamine resin, conventionally relatively low molecular weight water-soluble melamine resins excelling in water dispersibility have been dominantly used, but investigations for using hydrophobic melamine resins having relatively high molecular weight are being made recently. Water-based paint in which hydrophobic melamine resin is used as the crosslinking agent has an advantage to provide coating film of better performance such as water resistance, compared to water-based paint containing water-soluble melamine resin as the crosslinking agent, but in certain cases is inferior in storage stability or coating film appearance. Solution of this problem is held to be a subject for further studies.
With the view to solve the problem, JP 2002-308993A discloses an aqueous resin dispersion containing a reaction product obtained by heat-treating specific acrylic resin, hydrophobic melamine resin and polyester resin under specific conditions excels in storage stability and can form coating film excelling in finished appearance, water resistance and the like. When the aqueous resin dispersion is blended with effect pigment such as aluminum flake, mica and the like and used as water-based metallic base coat paint, however, there rises a problem that the formed coating film may develop metallic mottling or shows inferior flip-flop property or smoothness.
JP Sho 63 (1988)-193968A discloses a water-based coating composition containing a crosslinking agent formed by dispersing hydrophobic melamine resin in water in the presence of water-soluble resin gives a water-based base coat paint composition exhibiting high storage stability and favorable coating workability, being free of such defect as sagging, unevenness or the like over broad humidity range. Whereas, coating film formed of the water-based coating composition is subject to a problem of inferior smoothness.
Furthermore, JP Hei 7 (1995)-41729A discloses that an aqueous resin dispersion prepared by dispersing graft resin and hydrophobic melamine resin in an aqueous medium excels in storage stability, heat stability, mechanical stability and the like. Coating film formed of water-based paint composition containing the aqueous resin dispersion, however, has the problem of tending to develop metallic mottling and being inferior in flip-flop property or smoothness, failing to provide satisfactory coating film appearance.